Why no 4 wheel drive on roads?

[CausticUrbanCoast]

smaller wheels rotate slower...so your wrong.

Okay

 

[SmallChange]

Funny to see this just now. I just filled one of those Bagster folding dumpsters with rotting fence panels and construction scrap, and approached it from the non-road side and picked it up by the handles with my pallet forks so I could tie the handles towards one another and be sure I satisfied the requirement that the lifting straps can meet in the middle. While it was hanging from the forks I decided I'd left it too close to the road, so I tried to back up in 2WD, but my rears slipped -- they're filled with 500 lbs, but I was not wearing my 800 lb ballast box. I put it in 4WD, which worked fine.

But then I forgot, and later (without the Bagster) made a tight turn onto my paved driveway. I startled myself with a vigorous case of wheel hop. So, there's a good reason. The tractor looks funny with wheel hop, and the startled driver looks even funnier.

 

[Farmer495]

The ft axel is designed to run slightly faster than the rear to allow the driver to shift to 4wd while moving at speed. If both axels were geared the same, you could not engage 4wd smooth. Awd vehicles have a limited slip between ft & rear end. On vehicles that have lock up function on awd they still have to be unlocked to drive on hard surfaces.

Tractor front axle runs faster to allow the tractor to steer better, if it didn't lead(run faster) the front end would slide and be pushed by the rear axle when turning on soft/slick surfaces.

Since gearing is "%" faster, the difference is greater the faster you drive, your 4wd engagement with be harsh or grind if mechanical (versus clutch pack) Try shifting mechanical box tractor into 4x4 in first gear versus road gear.

Kubota M-110 etc had "bi speed" front axle, it ran one ratio normal, but would upshift (automatic powershift) the front axle to a faster speed when you turned sharp. I drove one years ago when new, the 110 HP tractor would almost pivot on one rear wheel without using brakes.

Pickup truck front axle runs the same speed as rear...easy engagement at highway speed, even on dry pavement if you are going straight.
Pickup truck also doesn't like to turn as easily in snow or mud, especially towing a trailer....BTDT.

 

[Farmer495]

smaller wheels rotate slower...so your wrong.

To go the same travelled distance (when rolling) smaller wheels will turn more often then larger ones since they have less circumference.

If the driven axle shaft is the same rpm, a small wheel will rotate slower, but that would mean on a tractor the rear axle is pushing skidding/locked up front tires. The front tire shaft RPM has to be increased to make them travel the same, or slightly farther (oversped) distance. So yes, the front wheels end up rotating faster.

 

[ning]

smaller wheels rotate slower...so your wrong.

In case this isn't a troll despite appearances:
1: smaller wheels rotate more times per distance traveled than do larger wheels (ie, they rotate faster)
2: you can choose the final drive (differential) gearing to closely match the front and rear driveshaft speeds even when there's substantial wheel size difference.

The front wheels of my tractor are about ½ the diameter of the rear, yet the gearing is such that the front and rear driveshafts spin at almost the same speed, as evidenced that the tractor doesn't grind to a halt or wheel hop as soon as it hits pavement in 4wd.

 

[JWR]

smaller wheels rotate slower...so your wrong.

No, ed1978, that is not the case. As several posts have pointed out, the front wheel gearing on all 4WD tractors is designed to cause the front wheels to pull ahead slightly more inches per unit time than the rear. That is what Caustic... meant saying that the smaller (front) wheels "rotate at a faster rate than the rear ones." OK, maybe his description was worded imperfectly but he is correct. Assuming std size tires and wheels it has nothing in the world to do with the size of the tire. It is the intentional gearing driving those tires. That design is on purpose to cause the front wheels to pull ahead just slightly.

This entire topic is covered, if I may say so, IMHO as I see people say, by my post #59.

 

[coast40]

My 69 Bronco ended up with approximately 3.54 rear gears and 3.50 front gears. When I got it the ratios were about opposite; it was a bucking Bronco. For more confusion, Google Ackermann steering... https://en.wikipedia.org/wiki/Ackermann_steering_geometry

 

[4570Man]

No, ed1978, that is not the case. As several posts have pointed out, the front wheel gearing on all 4WD tractors is designed to cause the front wheels to pull ahead slightly more inches per unit time than the rear. That is what Caustic... meant saying that the smaller (front) wheels "rotate at a faster rate than the rear ones." OK, maybe his description was worded imperfectly but he is correct. Assuming std size tires and wheels it has nothing in the world to do with the size of the tire. It is the intentional gearing driving those tires. That design is on purpose to cause the front wheels to pull ahead just slightly.

This entire topic is covered, if I may say so, IMHO as I see people say, by my post #59.

He’s wrong on every level. The smaller wheels turn more RPMs to drive the same speed and the front wheels are geared to go faster as in distance traveled than the rears.

 

[CausticUrbanCoast]

No, ed1978, that is not the case. As several posts have pointed out, the front wheel gearing on all 4WD tractors is designed to cause the front wheels to pull ahead slightly more inches per unit time than the rear. That is what Caustic... meant saying that the smaller (front) wheels "rotate at a faster rate than the rear ones." OK, maybe his description was worded imperfectly but he is correct. Assuming std size tires and wheels it has nothing in the world to do with the size of the tire. It is the intentional gearing driving those tires. That design is on purpose to cause the front wheels to pull ahead just slightly.

This entire topic is covered, if I may say so, IMHO as I see people say, by my post #59.

He’s wrong on every level. The smaller wheels turn more RPMs to drive the same speed and the front wheels are geared to go faster as in distance traveled than the rears.

Not sure about every level, my engine only goes at one rpm.

 

[K5lwq]

Not sure about every level, my engine only goes at one rpm.

Interesting. All of my engines will go at different rpms depending on where I have the throttle set.